The Land-Sea Interactions
The Land-Sea Interactions
Continent-Sea Interface: a Hydrogeological Continuum
The section of land between continent and ocean is the hydrogeological continuum. Geological reservoirs containing groundwater resources do not stop at the shoreline. These formations are continuous and may be covered by less permeable formations on the continental shelf. This interface on the coast takes the form of a transition zone between freshwater and saltwater due to the difference in density, the geometry of aquifers and the heterogeneity of their physical properties as well as how the coastal aquifers are used (supply of drinking water, agriculture, tourism, industry, etc.) and also, as freshwater inputs into the ocean. The proper management of coastal aquifers inevitably involves the risk assessment of saltwater intrusion, based on geological and hydrogeological knowledge of formations, observation and alert networks, management models integrating geological, hydrogeological and geophysical data and technical management solutions. Using methods to characterize clastic sedimentary geological reservoirs by sequential stratigraphy and seismic stratigraphy both onshore and offshore , respectively, helps suggest appropriate management tools for coastal aquifers that develop within sedimentary basins. In addition, characterizing the paleo-geographic evolution of the development of carbonate platforms over the course of geological time associated with the changes in sea level and vertical tectonic movements is essential in order to establish conceptual models required for monitoring and managing karstic coastal aquifers. Airborne geophysics provides information about the structure and intrusion of saltwater in formations, both sedimentary and volcanic. Different examples of coastal aquifers, primarily Mediterranean, show the specificity of this continuum, characterization and monitoring tools as well as management tools.
Coastal zones form an interface between the land and the sea surface. Underground, coastal aquifers in turn form a hydrogeological continuum, an interface between the ocean and continent. Groundwater flows in geological formations, usually perpendicular to the coastline, toward continental and marine outlets on the continental shelf with the release of groundwater or occasionally underwater sources.
Groundwater is found in a variety of geological formations, between the sea and land: in detrital sedimentary formations (deltas, sedimentary basins, alluviums, etc.), in karstified limestone formations (Mediterranean, Mexico, Florida, etc.), bedrock formations (Britain, Scotland, Scandinavia, etc.) or volcanic formations (Reunion, Mayotte, Caribbean, Canaries, Azores, etc.). This space between the continent and ocean is an interface between salt and freshwater. Saltwater can penetrate the continent depending on natural conditions and abstractive conditions. As its density is higher than that of freshwater, it forms below freshwater, which we call a saltwater intrusion (theoretical abrupt interface) or a transition zone (diffuse interface).
This continent-ocean interface is a zone of interest both for human societies, with more than 60% of the world's population living on the coastline less than 60 km wide, and for lagoon and marine ecosystems. This coastal zone attracts populations and tourists, both due to climatic conditions and quality of life as well as economic development (sea transport, import-export). Water resources are necessary for the development of this zone. Groundwater in aquifers of this sea-land continuum is an important source both for the supply of drinking water and for human activities (agriculture, industry, tourism, etc.). Specific ecosystems develop in the ocean or in lagoons next to groundwater outlets and may also be zones of economic interest (fishing, oyster farming, fish farming, etc.).